Current Transducer

Thread Starter

scorca

Joined Oct 31, 2008
36
Hi all,

We are building a MPPT charger for solar panel rated at 300 W, 15 V @ 20 A.
At the measuring part, we use a current transducer (ct) LEM HXS 20-NP/SP 3 (datasheet:http://www.farnell.com/datasheets/61020.pdf). We have it at the input and output. At the input has to measure about 20 A and at the output has to measure about 5-6 A. Somehow, we tested the ct and unfortunately the output of ct was not changing when the current was changing. We tested 3 same ct and those gave me the same output. Does anybody know why? Thank you in advance.


Regards,
Ben
 

eblc1388

Joined Nov 28, 2008
1,542
Describe or post your CT test circuit. What voltages or current are connected to which pin?

Have you connected the three capacitors shown in image? Vref is the internal reference voltage and can be replaced by an external one if required but in your case, only a single 47nF capacitor is needed.



I have not used this type of transducers before but someone else must have so they can give you suggestions.
 

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italo

Joined Nov 20, 2005
205
Since you are asking question I wander if the output was ties to any load? Without a load you will never get any current to register.
 

eblc1388

Joined Nov 28, 2008
1,542
Referring to the formula for the CT output as given in the datasheet:



Ipn depends on the input connections configuration and in your case the CT is wired up for 20A nominal. Thus Ipn=20A in the formula.

At 2amp primary current, CT Vo=2.5V-(0.625*Ip/Ipn) = 2.5 - 0.625*2/20 = 2.4375V.

If you reduce the primary current to 1A, the output voltage = 2.5 - 0.625*1/20 = 2.4688V. The difference is only 0.03V. Some DVM will automatically changes to use the 20V range to display an input voltage larger than 2V. You would need a DVM with a range other than fsd 20V to actually see the change more clearly.

If you have connected a DVM to the output of the CT in your previous test, does the reading not change, even in the last digit?
 

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Alberto

Joined Nov 7, 2008
169
From your schematic connection are wrong!.
Input should be on the side you are using as output (see spec under remark). So very likely you have a negative output.
Just reverse connection and it should work.

Alberto
 

Thread Starter

scorca

Joined Oct 31, 2008
36
When there is no Ip, the output of my ct shows 7.4 mV only. It doesnt show 2.5 V.
And when we increased the Ip, the output of the ct was increasing. But when we decreased the Ip, the output of the ct was still increasing slowly.
 

eblc1388

Joined Nov 28, 2008
1,542
When there is no Ip, the output of my ct shows 7.4 mV only. It doesnt show 2.5 V.
There is something not right here.

Have you disconnected all the loading on the CT output before measuring the voltage? The datasheet requires CT output loading be larger than 2KΩ.
 

eblc1388

Joined Nov 28, 2008
1,542
I tested also another current transducer HX 20-P/SP2 (datasheet:http://www.farnell.com/datasheets/118834.pdf) and it was working (without Vref). I used 12 ohm Rload. The datasheet says it has to be > 2 kOhm.
If one does not follow the limits stated explicitly in the datasheet, then the part will not perform correctly or satisfactorily or will end up having a short service life.

Say goodbye to your CTs if you insists on using 12Ω output loading. You might have already killed them. :(
 
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